Fuel injectors are used to direct fuel into a combustion chamber. Normally inside a fuel injector, fuel is pressurized, and a nozzle assembly including a nozzle valve (nozzle needle valve) is used to control fuel flow through nozzle orifices. At “off” position, the nozzle valve blocks fuel flow. When the nozzle valve moves to “on” position, fuel is injected through the orifices. The overall fueling rate is controlled either by pre-metering fuel (e.g. unit injectors) or controlling the injector “on” time (e.g. common rail systems).
Due to the high pressure high temperature working environment in fuel injectors, it is hard to measure the fueling flow rate directly. As a result, fueling rate control for most injectors is open-loop feed-forward control. Control error and injector deterioration may cause poor combustion and emission issues.
Air-fuel ratio (lambda) control can be used for adjusting fueling rate indirectly. However, in air-fuel ratio control, the goal is to control air-fuel ratio rather than fuel injection rate. In the control system, therefore, the whole fuel system including fueling rate control modules is part of control plant to the air-fuel ratio controller. Performance change in fueling rate control, especially that caused by fuel system deterioration is a perturbation to the control system rather than a disturbance, causing deterioration in air-fuel ratio control.
Additionally, lambda sensors normally are positioned at the downstream of exhaust manifold. Therefore, the adjustment of fueling rate actually is for all cylinders rather than individual ones. Fuel injector deterioration in some cylinders may cause over or under fueling in other cylinders, resulting in fuel economy, torque balance, and emission issues.
The shape of fueling pulses is important to combustion. Normally, fueling pulse shape can be controlled either by modulating fuel pressure or changing injector geometry during fuel injection. In common-rail systems, fuel pressure is kept constant, resulting in that fuel pulse shape can only be controlled by adjusting injector geometry. However, injector geometry change could deteriorate fuel atomization and penetration, causing combustion and emission issues. In pre-metered systems, fuel pressure is applied with the movement of engine camshaft. On one hand, it is relatively easier to modulate the pressure for controlling injection pulse shape. On the other hand, however, the injection pulse shape is strongly affected by engine camshaft speed.
Fuel systems, especially systems in Cl (Combustion Injection) engines, must be highly reliable. In common-rail systems, a high constant fuel pressure is maintained. If a malfunction causes an injector valve being stuck open, fuel could be injected into combustion chamber continuously, causing catastrophic results. In pre-metered systems, though a stuck open injector won't lead to continuous fuel injection, losing pre-metering control could still cause ill combustion, emission, and safety issues.
To solve the drawbacks of common-rail systems and pre-metered systems, a fuel system needs to have real-time feedback control and flexible fueling shape control with fuel pressure modulated independently. The fuel system should also be highly reliable. Malfunctions such as valve being stuck open should not cause emission and safety issues.